1. Field of the Invention
This invention relates to a laser recording apparatus capable of reproducing image information such as characters and figures from a computer, an original reading apparatus, a word processor or the like at a high speed, and more particularly to a laser recording apparatus for effecting recording of high quality at high speed by deflecting and modulation-controlling a laser beam by image information such as figures and characters from a computer, an original reading apparatus, a word processor or the like.
2. Related Background Art
In a laser recording apparatus such as a laser beam printer, a laser beam is generated by a semiconductor laser or the like to form record images on a photosensitive member which is a recording medium, and the emission timing of this laser beam is taken by emitting said beam to the reflecting surface of a rotating polygon mirror provided at the front stage on the emission side of the semiconductor laser, and detecting a beam at a side edge portion of the laser beam deflected and scanned toward the photosensitive member. For the detection of this beam, the side edge portion of said beam is applied to a return mirror for a horizontal synchronizing signal which reflects the beam, and the beam from this return mirror for a horizontal synchronizing signal is directed to a horizontal synchronizing signal detecting element to thereby take the timing.
FIG. 1 of the accompanying drawings shows a laser recording apparatus disclosed in Japanese Laid-Open Patent Publication No. 61-13759.
In FIG. 1, a light beam emitted from a semiconductor laser 101 passes through a collimator lens 102a and is collimated thereby, and is reflected by a polygon mirror 103 which is a deflector as scanning means. The polygon mirror 103 is rotated at a predetermined speed in the direction of arrow by a motor, not shown. With the rotation of the polygon mirror 103, the reflected light beam is scanned, and a passes through an f.theta. lens 104 as a scanning lens and is imaged on an electrophotographic photosensitive drum 105 which is a recording medium, and is scanned in the direction of arrow 105a. On the drum 105, exposure to a light beam modulated correspondingly to information to be recorded is started from a point 105b by means which will be described later.
That is, the image forming area of the photosensitive drum 105 is scanned by this light beam. Designated by 102b and 102c are cylindrical lenses disposed forwardly and rearwardly of the polygon mirror 103. The cylindrical lenses 102b and 102c together constitute a tilt compensating optical system for making the scanning locus of light beam constant on the drum 105 even if the rotary shaft of the polygon mirror 103 is inclined.
Before the light beam scanned by the polygon mirror 103 and passed through the lens 104 arrives at the drum exposure starting point 105b, the light beam is reflected by a mirror 106 and moves in the direction of arrow 106a. Thus this light beam scans the entrance end surface 108b of an optical fiber 108a through the slit of a mask 107 and an auxiliary lens 115 having a condensing property. When the light beam enters the entrance end surface 108b, the light thereof is transmitted by the optical fiber 108a, and a signal is formed by a detecting element 108 such as a PIN photodiode or CdS (cadmium sulfide). The signal is amplified by an amplifier 109 and the point of time at which this amplified signal has risen to a predetermined level is detected by a detector 110. In a predetermined time after this point of time, a timer 111 operates a memory 112 and causes the memory 112 to transmit an information signal to be recorded to a driving circuit 114. (An information signal to be recorded is imparted in advance to the memory 112 from a signal source 113 such as a computer, an original reading apparatus or a word processor and is stored therein.) The driving circuit 114 drives the semiconductor laser 101 in response to said recorded information signal and thus, the semiconductor laser 101 emits an ON-OFF-modulated laser beam correspondingly to said signal.
FIGS. 2A and 2B are enlarged views of the horizontal synchronizing signal detecting portion of the above-described prior-art laser recording apparatus, and FIG. 2A represents the state of the light beam on the optical path in the main scanning plane (a plane parallel to the deflecting surface), and FIG. 2B represents the state of the light beam in a direction orthogonal thereto, i.e., the auxiliary scanning direction. The light beam for a horizontal synchronizing signal returned by the return mirror 106 passes through a slit 107 located substantially at the same optical path length as the drum surface. This slit 107 is used to increase the sensitivity of timing of the beam in the main scanning direction. The light passed through this slit 107 passes through a condensing lens 115 and arrives at the entrance end surface of an optical fiber 108. A photodetector element is provided on the other end surface of the optical fiber, and photoelectric conversion is effected thereon. The return mirror 106 and the entrance end surface of the optical fiber are in optically conjugate relationship with the condensing lens 115 so that even if the reflecting mirror 106 is more or less inclined in the auxiliary scanning direction as indicated, for example, by broken line in FIG. 2B, the light beam for a horizontal synchronizing signal will not deviate from the entrance end surface of the optical fiber to make photodetection impossible. However, the optical path length from the mirror 106 to the condensing lens 115 need be substantially the same as the ideal optical path length to the photosensitive drum, and this has led to the disadvantage that it is difficult to draw around the optical path and therefore the entire optical scanning apparatus becomes bulky.
Also, as is disclosed in Japanese Laid-Open Patent Publication No. 62-175612, a method of shortening the distance to the light receiving surface is proposed, and a schematic view of the detecting portion thereof is shown in FIG. 3 of the accompanying drawings. The light beam for a horizontal synchronizing signal returned by the return mirror 106 passes through a cylindrical lens 38 having a power in the main scanning direction and having no power in the auxiliary scanning direction. Further, the light beam passed through this lens is condensed by a cylindrical lens 39 having no power in the main scanning direction and having a power in the auxiliary scanning direction and arrives at a photoelectric converting element 40. Here, the cylindrical lens 38 has the function of shortening the imaging position of the light beam for a horizontal synchronizing signal, and the cylindrical lens 39 has the function of making it difficult for the light beam to deviate from the photoelectric converting element relative to the movement of the return mirror 37 in the auxiliary scanning direction and the function of imaging the light beam in the auxiliary scanning direction. In this system, however, the light receiving surface of the photoelectric converting element is placed at the point whereat the laser beam is condensed by the condensing lens, that is, the mirror 106 and the light receiving surface of the photoelectric converting element 40 are not in optically conjugate relationship with the condensing lens. This leads to the problem that due to the inclination of the reflecting mirror, photodetection becomes difficult.